Move validation of compiled unit out of compiler

Instead of having the compiler do any validation on the
graph it has created instead have the compiler just compile
and have the engine that uses that compiled result do any
post compilation validation instead.

This makes it more clear that the compiler just compiles a
flow (and tasks and nested flows) into a graph, and that is
all that it does.

Change-Id: I96a35d732dc2be9fc8bc8dc6466256a19ac2df6d

taskflow/utils/misc.py

@@ -35,7 +35,6 @@ from oslo_utils import importutils
 from oslo_utils import netutils
 from oslo_utils import reflection
 import six
-from six.moves import map as compat_map
 from six.moves import range as compat_range
 
 from taskflow.types import failure
@@ -453,18 +452,6 @@ def sequence_minus(seq1, seq2):
     return result
 
 
-def get_duplicate_keys(iterable, key=None):
-    if key is not None:
-        iterable = compat_map(key, iterable)
-    keys = set()
-    duplicates = set()
-    for item in iterable:
-        if item in keys:
-            duplicates.add(item)
-        keys.add(item)
-    return duplicates
-
-
 class ExponentialBackoff(object):
     """An iterable object that will yield back an exponential delay sequence.